In one conventional gas sensor attached to an exhaust system such as an exhaust pipe of an engine to detect the concentration of a specific gas component in the exhaust gas, a heater and a detection element including at least one cell in which a pair of electrodes are disposed on surfaces of a solid electrolyte body are stacked and integrated together.
In the gas sensor configured as described above, the solid electrolyte body is exposed at side surfaces of the detection element, and electrically conductive materials such as soot contained in the exhaust gas can adhere to the exposed portion of the solid electrolyte body. The solid electrolyte body has a portion which is heated to a temperature which is lower than the temperature at which the soot burns out (about 600° C.) and at which the solid electrolyte body exhibits oxygen ion conductivity (e.g., 200 to 600° C.). If soot adheres to such a portion, the soot causes a leakage current, and the gas concentration detection performance of the gas sensor deteriorates.
One technique developed to address this issue is to apply a paste composed mainly of alumina to the exposed portion of the solid electrolyte body, which is heated to a temperature lower than 600° C. during the use of the gas sensor element, to thereby insulate the exposed portion (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-250925).
When the alumina paste is applied to the solid electrolyte body by printing, pinholes are formed. One technique developed to address this issue is to insulate the exposed portion of the solid electrolyte body using, instead of alumina, a glass coating having a glass transition point higher than 700° C. (see, for example, Japanese Patent No. 5638984 (FIG. 2)).